An image reading section is installed in apparatuses such as a facsimile machine, a copying machine and an image scanner serving as a peripheral device of a personal computer. In such an image reading section, a method is adopted in which an original image is read while a reading member is being transported relative to an original document as described above. Here, in the image reading section, normally, the discrimination F (mm/step), which corresponds to the amount of feed of the original document per 1 step of a feeding motor, and the resolution P (mm/line) of the reading member are coincident with each other. Therefore, in an apparatus such as the above-mentioned facsimile machine, when the feeding motor makes one revolution (step), the reading member also carries out one reading operation for one (line).
However, alternation of the resolution P is demanded, for example, in cases such as the standard-letter mode and minute-letter mode in a facsimile machine. In order to meet such a demand, for example, Japanese Laid-Open Patent Publication No. 75780/1993 (Tokukaihei 5-75780) has disclosed a typical conventional technique. In this conventional technique, when a reading operation is carried out by using the resolution reduced to 1/n of the maximum resolution, respective "n" number of adjacent CCDs (Charge Coupled Devices) serving as the reading members are connected in parallel with one another so as to addition-combine the outputs of the pixels.
Therefore, the problem with this conventional technique is that it is not applicable unless the desired resolution P is set at an integral multiple of the discrimination F.
Moreover, Japanese Laid-Open Patent Publication No. 145715/1993 (Tokukaihei 5-145715) is listed as another conventional technique. In this conventional technique, supposing that the normal transport speed is V, a transporting operation is carried out at a low speed in the range of V/2 to V, and a predetermined number of scanning lines are disabled depending on the transporting speed.
The subsequent problem with this conventional technique is the occurrence of image distortion due to the disabled scanning lines.
Similarly, Japanese Laid-Open Utility Model Publication No. 191261/1987 (Jitsukaishou 62-191261) has disclosed another conventional technique in which lines that have been read are counted and thinned out so as to fit the resolution on the playback side.
Therefore, the subsequent problem with this arrangement is also the occurrence of image distortion due to the thinning-out process on the above-mentioned lines.
In order to solve these problems, Japanese Laid-Open Patent Publication No.257382/1988 (Tokukaishou 63-257382) has disclosed a conventional technique which can change the resolution P while suppressing image distortion even if the resolution P is not set at an integer multiple of the discrimination F. In this conventional technique, after a reading operation has been carried out with the discrimination F of the transporting motor being set at a predetermined value, conversion to a desired resolution P is made by operation processing on a software basis.
Furthermore, another conventional technique proposes a method in which supposing that the reading speed of a reading member is Vp (msec/line), image data of the entire original document is converted by using a buffer memory so that the transporting speed Vf (msec/step) of a transporting motor can satisfy the following equation (1): EQU Vf.times.j=Vp.times.k (1),
where j and k are arbitrary integers.
In addition, in the above-mentioned technique, another modified method is proposed in which in the case when, upon using the above-mentioned equation (1), the capacity of the buffer memory has an image-data amount less than one page of an original document, the reading operation of the reading member is temporarily stopped when the buffer memory becomes full.
However, the method using the operation processing on a software basis raises the following problems: an operation processing circuit is required, image distortion occurs due to the operation processing, and the time required for the operation processing gives adverse effects on the entire reading time.
Moreover, another problem with the method using the buffer memory is that a large memory, that is, a memory large enough to store the amount of image data corresponding to at least one page of original document, is required in order to carry out a smooth reading operation, thereby resulting in high costs.
Furthermore, in the method in which the reading operation is stopped when the buffer memory becomes full, upon stoppage of a reading operation, an accumulated offset takes place which is equivalent to the number of lines that have been read and which corresponds to the difference between the discrimination F of the transporting motor and the resolution P of the reading member. This results in an image distortion corresponding to one line, in the very worst case.